The performance of personal computer systems has improved as the technology has improved. Most notably, the speed of the central processing systems has greatly increased. To keep up with the increased processing speeds of the microprocessors, computer bus systems have also had to evolve.
The original IBM.RTM. personal computer (the IBM PC XT) had a slow 8 bit bus know as the XT bus. The XT bus was abandoned by IBM in their second generation personal computer (PC) known as the IBM.RTM. PC AT. The IBM.RTM. PC AT had a sixteen bit wide bus known as the AT bus. Soon after the IBM.RTM. AT was introduced, a large number of computer makers introduced personal computers that were compatible with the IBM.RTM. PC AT. The "IBM.RTM.-compatible" computers had a bus that functioned the same as the AT bus.
IBM.RTM. then introduced a newer computer bus known as the Microchannel.RTM. bus. The IBM.RTM. Microchannel.RTM. was a proprietary bus and IBM.RTM. required royalty payments from any computer maker that implemented the IBM.RTM. Microchannel.RTM. bus. Instead of adopting the newer Microchannel.RTM. bus, the compatible computer manufacturers kept using the older AT bus and renamed it the Industry Standard Architecture (ISA) bus. The Industry Standard Architecture (ISA) bus became a de facto standard for personal computers.
As processor speeds increased, it became apparent that the sixteen bit wide ISA bus was becoming a bottleneck for computer performance. To remedy this situation, new thirty-two bit wide bus standards were proposed including the VESA local bus (VLB) and the Peripheral Component Interconnect (PCI) bus. Both standards were made freely available. The Peripheral Component Interconnect (PCI) bus has become the de facto industry standard.
A large number of peripheral cards still exist for the ISA bus. For example, many sound cards, such as the SoundBlaster.TM. (a trademark of Creative Labs, Inc.) sound card that dominates the industry, are ISA based cards. Thus, most computers still contain an ISA bus in order to use such ISA cards. However, the ISA bus is implemented in addition to the more advanced PCI bus. In fact most current computer systems typically have three distinct buses, namely, a processor (host) bus, a Peripheral Component Interconnect (PCI) bus and an Industry Standard Architecture (ISA) bus. The host bus is a very fast bus that is directly connected to the processor (host) of the computer system. The PCI bus is coupled to the host bus through a host bridge. The slow ISA bus is coupled to the PCI bus through an expansion bus bridge.
A new computer bus standard, known as the Universal Serial Bus (USB), is currently being adopted by many computer manufacturers. The Universal Serial Bus (USB) is a bus that allows external peripherals to be added to a computer system. The USB is very simple to use since USB devices are connected together in a simple daisy chain system.
One feature of the Universal Serial bus is the ability to perform isochronous transfers. To perform isochronous transfers properly, the Universal Serial Bus requires guaranteed access to the computer memory. Thus, it would be desirable to have an efficient method of adding the Universal Serial Bus (USB) to personal computer systems that will supply the guaranteed access.
Furthermore, it is very desirable to keep the number of integrated circuit parts needed for a personal computer low. This can be accomplished by combining several different functions into a single integrated circuit. Nearly every IBM.RTM.-compatible personal computer uses an external device interface known as the Integrated Drive Electronics (IDE) interface. The IDE interface is used to connect hard disk drives and CD-ROM devices to the computer system. Thus, it would be also be desirable to integrate an IDE interface controller with a USB interface controller.